The present invention relates to a continuous process for impregnating fibers and its implementation device.
It is already known how to continuously impregnate fibers by different techniques such as passing the fiber through the polymer in a melted state; passing the fiber through a liquid solution or suspension containing the polymer; or passing the fiber through a suspension of the powdered polymer in a gas.
Such operations are then followed by a heat treatment producing the polymer in a state which permits the penetration of the latter into the fiber and shaping operations such as molding.
Now, the first two techniques have at least one of the following disadvantages, in particular when the polymer is thermoplastic:
uneven and non-homogeneous sheathing of the fiber leading subsequently, during heat treatment, to an uneven and non-homogeneous impregnation and, consequently, to mediocre properties especially with aging; PA1 need for eliminating the liquid which has been used for putting the polymer in a solution or in suspension, this elimination being able to be carried out only with appropriate heating devices also optionally combined with recovery devices, or even recycling devices; PA1 difficulty in getting rid of residual traces of liquid and formation of pores which are detrimental to a good adherence between the fibers and polymer matrix and, therefore, to good mechanical properties; or PA1 delicate control of the respective weight or volume ratio of polymers and fibers. Now, such a control is a determining factor in the operation of a continuous process for impregnating fibers.
The technique of impregnation by passing the fiber through a suspension of powdered polymers in a gas must be separated into two categories according to the nature of the suspension, namely, aerosols and fluidized beds.
These two forms of suspensions where particles or micro-particles of powder are momentarily dispersed in a gaseous continuum have particular characteristics. Amongst others, the fluidized bed has a horizontal upper surface similar to a liquid in a container. The concentration of a fluidized bed relative to the volume that it occupies has no meaning, as it does for a liquid, unlike an aerosol.
The impregnation of fibers in a fluidized bed has the same disadvantages as regards the control of the polymer weight or volume ratio taken up by the fibers as impregnation in a liquid suspension. This ratio being a function, separately or simultaneously, of the duration of immersion in the solution, the liquid suspension or the fluidized bed; the concentration of the solution or of the liquid suspension; and the efficiency of the device used to eliminate the excess polymer taken up.
When it is necessary to vary the duration of the immersion, it is, in general, the geometry and the trajectory of the fiber in the impregnation medium which must be modified if the speed at which it passes through is not changed. The constant maintenance of the ratio depends on the homogeneity and the concentration of polymers of the impregnation medium and of the maintenance of these. The creation of a fluidized bed is theoretically possible with all powders, but has some practical difficulties. Certain polymers which are very finely powdered and lumpy, such as novolak phenolic resins, fluidize very poorly. Further, powdered polymers which are subjected to treatment by a liquid or have a small percentage of liquid added to them (which is common practice with powdered thermosetting plastics) cannot be fluidized or put in liquid suspension or in solution.
On the other hand, these powders which are difficult to fluidize can be put in an aerosol suspension without modification of the initial formulation or granulometry.
The known processes using an aerosol suspension as an impregnation medium propose devices for regulating the concentration which thus permits the quantity of polymer powder taken up by the fiber to be controlled. All these processes require an adjustment which links the overall concentration in the impregnation chamber of the aerosol suspension and the polymer - fiber take up ratio. This adjustment must be redone after each change of fiber or polymer type and according to the capture conditions of the fiber. Among these impregnation processes in an aerosol medium which require this adjustment, there must be mentioned those which generate the aerosol suspension by drawing the impregnating medium into a separate fluidized bed. The recycling of the non-fixed polymer powder of the aerosol part into the fluidized bed does not alone exclude the need for adjustment.
It would, therefore, be desirable to have at one's disposal a process for impregnating fibers with a polymer powder, the process not requiring adjustment yet providing for a very even impregnation. Furthermore, it would be desirable to have a process which will carry out this impregnation at high speed and at very low cost.